In an LBP and PPC, laser light from a semiconductor laser is irradiated over a photosensitive drum in accordance with the video data, such as letters and picture image data, and the aforementioned video data is printed by using a toner by the condition of electrification in accordance with the condition of irradiation.
Semiconductor lasers that are adopted in such an LBP, for example, include anode stem types, in which the anode of the semiconductor laser diode is connected to the power source side, and the current drive circuit is connected to the cathode, and cathode stem types, in which the cathode of the semiconductor laser diode is connected to the power source (ground) side, and the current drive circuit is connected to the anode side.
Of these 2 types of semiconductor lasers, cathode stem type semiconductor lasers are often used from the standpoint of design and reliability, for example.
FIG. 2. is a circuit diagram which shows the structural example of a conventional current drive circuit in a cathode stem type semiconductor laser.
This current drive circuit DRV consists of a drive current switching circuit 10, bias current supply 20, data input circuit 30, and a semiconductor laser 40.
The drive current switching circuit 10 supplies or stops the supply of the drive current ISW to the semiconductor laser 40 in accordance with the data that is input through the data input circuit 30.
Also, the bias circuit 20 supplies the bias current Ib to the semiconductor laser 40.
The drive current ISW and the bias current Ib are supplied to the current supply terminal NDLD, which is a connecting node of the drive current switching circuit 10, bias current supply 20, and the semiconductor laser 40.
In the current drive circuit DRV in FIG. 2, the drive current switching circuit 10 consists of an op amp OP101, npn transistors N101-N104, pnp transistors P101-P103, current-setting resistor R101, and diodes D101 and D102 that are connected in series.
Also, the bias current 20 consists of an op amp OP201, npn transistors N201-N204, pnp transistors P201-P203, and a current-setting resistor R201.
Also, the data input circuit 30 consists of npn transistors N301-N305, pnp transistor P301, Zener diode ZD301, and resistors R301-R305.
Also, the semiconductor laser 40 consists of a laser diode LD401, photodiode PD401, and a resistor R401.
In this current drive circuit DRV, the value of the drive current ISW is determined by the voltage VIN that is applied to the input terminal TIN1 of the drive current switching circuit 10 and the resistance of the current-setting resistor R101. The current which is generated at the input stage in this manner is supplied to the bases of the drive current ISW supply transistors N102 and N103 through the transistors P101-P103 and N104 which make up a current mirror circuit.
Also, the value of the bias current Ib is determined by the voltage VIN2 that is applied to the input terminal TIN2 of the bias circuit 20, and the resistance of the current-setting resistor R201. The current generated at the input stage in this way is supplied to the bases of the bias current Ib supply transistors N202 and N203 through the transistors P201-P203 and N204, which make up a current mirror circuit. In this way, the bias current Ib is supplied to the current supply terminal NDLD.
Thus, the drive current switching circuit 10 supplies or stops the supply of drive current ISW to the current supply terminal NDLD accordance with the input level of the stream signal/DT (where/indicates active low) of the video data, for example, to the data input circuit 30.
More precisely, since video data is input at a high level, the output node ND30 of the input circuit 30 is connected to the ground potential Vss. Accordingly, the cathode side potential of the diode D102 is below the anode side potential of the diode D101, which is at the base potential of the drive current supply transistors N102 and N103 of the drive current switching circuit 10. As a result, the diodes D101 and D102 are turned on, the bases of the transistors N102 and N103 are pulled to ground potential Vss, the transistors N102 and N103 are turned off, and the supply of the drive current ISW to the current supply terminal NDLD is stopped.
Accordingly, in this case, only the bias current Ib is supplied to the semiconductor laser 40 and it is maintained in a turn-off bias condition.
On the other hand, when video data is input at a low level, the output node ND30 of the input circuit 30 is maintained at the level of the power supply voltage Vcc. As a result, the diodes D101 and D102 are turned off, the transistors N102 and N103 are turned on, and the drive current ISW is supplied to the current supply terminal NDLD.
Accordingly, in this case, the current ISW+Ib is supplied to the semiconductor laser 40, and, as a result, the laser diode LD401 emits light.
However, in the conventional drive circuit in the cathode stem type semiconductor laser described above, the semiconductor laser could not be rapidly driven from a turned-off state by the current Ib when the drive current ISW was supplied, while controlling the transistors N102 and N103, to a turned-on state by the stream signal of the video data, the emission characteristic became extremely dull, and achieving a high-speed drive was difficult.
The aim of the present invention, in which such a circumstance is considered, is to offer a semiconductor laser drive circuit in which the semiconductor laser can be driven at high speed.